<template>
    <ul class="list">
        <li class="item" v-for="item of letters"
        :key="item"
        :ref="item"
        @click="handleletterClick"
        @touchstart="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
        >
        {{item}}
        </li>
    </ul>
</template>

<script>
export default {
  name: 'CityAlphabet',
  props: {
    cities: Object
  },
  computed: {
    letters () {
      const letters = []
      for (let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
  },
  data () {
    return {
      touchStatus: false,
      startY: 0,
      timer: null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods: {
    handleletterClick (e) {
      this.$emit('change', e.target.innerText)
    },
    handleTouchStart () {
      this.touchStatus = true
    },
    handleTouchMove (e) {
      if (this.touchStatus) {
        if (this.timer) {
          clearTimeout(this.timer) // 节流设置 16ms时间内如果有变化就清空上一个操作，提升性能
        }
        this.timer = setTimeout(() => {
          const touchY = e.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY) / 20)
          if (index >= 0 && index <= this.letters.length) {
            this.$emit('change', this.letters[index])
          }
        }, 16)
      }
    },
    handleTouchEnd () {
      this.touchStatus = false
    }
  }
}
</script>

<style  lang="stylus"  scoped>
  .list
   display: flex
   flex-direction: column
   justify-content: center
   position: absolute
   top: 1.58rem
   right: 0
   bottom: 0
   width:.4rem
   .item
    text-align: center
    line-height: .4rem
    color: #00bcd4
</style>
